In a surface-emitting laser array in which surface-emitting laser elements are integrated, the output of each surface-emitting laser element during operation may decline due to a temperature rise through reception of heat from the surrounding surface-emitting laser elements, and the life of the surface-emitting laser array may be shortened.
To obviate the problem, it is necessary to improve the heat dissipation characteristic. For example, a material having a high thermal conductivity should be used for a semiconductor Bragg reflector which is located on the side of main heat dissipation. Among the materials which can be used for a semiconductor Bragg reflector of a surface-emitting laser element on a GaAs substrate, AlAs is a suitable one that has the highest thermal, conductivity.
However, there is a case in which etching into the shape of a mesa structure is performed in order to separate the surface-emitting laser element from the surrounding part electrically or spatially. In this case, although it is unnecessary that the etching reaches the lower semiconductor Bragg reflector arranged on the substrate side, the design is carried out, from the problem of the controllability of etching, by assuming that the etching bottom reaches the lower semiconductor Bragg reflector.
For example, in the case of an oxidization type surface-emitting laser element, it is necessary to etch more deeply than a selective oxidation layer in order to perform selective oxidation. For the purpose of stopping the divergence of the current, it is common to arrange the selective oxidation layer at a place near the active layer of the p-type semiconductor Bragg reflector (or the semiconductor Bragg reflector arranged above the active layer), or in the position of the first to fifth nodes (nodes in the field strength distribution of a laser beam) from the active layer.
However, from the problem of the controllability of the etching depth, it is difficult to control the etching bottom to be deeper than the selective oxidation layer but not to reach the lower semiconductor Bragg reflector.
In order to control the etching depth in the whole wafer surface, it is necessary to not only control the etching time but also attain the uniformity of etching in the wafer surface, and the uniformity of distribution of thickness of the crystal growth layer. Practically, it is very difficult to carry out mesa etching so as be deeper than the selective oxidation layer but not to reach the lower semiconductor Bragg reflector.
To resolve the problem, Japanese Laid-Open Patent Application No. 2002-164621 discloses separating the lower semiconductor Bragg reflector into two layers. In the laser array of Japanese Laid-Open Patent Application No. 2002-164621, the substrate side one of the two lower semiconductor Bragg reflector layers is a main refractive index layer which is made of AlAs. AlAs has a thermal conductivity which is much larger than that of AlGaAs. On the other hand, the active-layer-side reflector layer is made of AlGaAs which is used conventionally.
However, in the case of the surface-emitting laser array, carrying out uniform mesa etching within the surface of the wafer is still more difficult for other additional reasons. If the inter-element gap of the surface-emitting laser elements is narrowed in order to carry out array arrangement with high density, a difference Δd between an etching depth of the inter-element gap and an etching depth of the flat part in the circumference of the surface-emitting laser array becomes large. Furthermore, a skirt portion arises in the etching configuration. It is desirable that the selective oxidation layer does not start from the skirt portion, in order to control the oxidization narrowing dimensions strictly.
However, if etching is performed so that the selective oxidation layer may not start from the skirt portion, the etching bottom in the flat part in the circumference of the surface-emitting laser array enters the lower semiconductor Bragg reflector.
Since the low refractive index layer of the lower semiconductor Bragg reflector usually is larger in thickness than the selective oxidation layer, if both the layers have the same composition, the oxidation rate of the low refractive index layer is quicker than that of the selective oxidation layer.
If the oxidation rate of the low refractive index layer of the lower semiconductor Bragg reflector is quicker than the selective oxidation layer, the whole low refractive index layer is oxidized early and performing the current pouring is impossible.
To avoid the problem, AlAs cannot be used as a material of the low refractive index layer which is located near the active layer of the lower semiconductor Bragg reflector. For this reason, in order to decrease the oxidation rate of the semiconductor Bragg reflector, it has been necessary to use AlGaAs in which a certain amount of Ga is added (for example, Al0.9Ga0.1As). Refer to the Technical Report CS-3-4 (2004) from the Institute of Electronics, Information and Communication Engineers, Electronics Society Convention, and IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, No. 12, 1999, pp. 1539-1541.
Japanese Laid-Open Patent Application No. 09-018093 discloses that etching of the upper semiconductor Bragg reflector is stopped up to a GaInP cladding layer (resonator region).
FIG. 37 is a plan view of a surface-emitting laser array according to the related art. As shown in FIG. 37, the double dummy elements are arranged in the circumference of the central array part where the surface-emitting laser elements are arranged.
Japanese Laid-Open Patent Application No. 2000-114656 discloses that the post (mesa) in the central array part and the post (mesa) in the circumference of the array part are subjected to a different environment and the posts (mesa) have a different configuration accordingly.
And Japanese Laid-Open Patent Application No. 2000-114656 discloses a surface-emitting laser array in which the double dummy elements arranged in the circumference of the central array part allows uniform characteristics.
In a conventional oxidization type surface-emitting laser array, the etching bottom in the flat part in the circumference of the surface-emitting laser array faces the lower semiconductor Bragg reflector. A material having a high thermal conductivity, such as AlAs, is easily oxidized if it appears on the surface by etching. AlAs cannot be used for the lower semiconductor Bragg reflector (at least in the region near the active layer).
Therefore, heat is easily accumulated in the active layer and the temperature of the active layer increases. There are the problems that the optical output declines and the life of the surface-emitting laser element becomes short. Especially, when the surface-emitting laser array operates, undesired influences due to thermal interference become remarkable. Operation of the surface-emitting laser array at high current values is impossible and use of the surface-emitting laser array with a low optical output is unavoidable. And the life of the surface-emitting laser array becomes short due to a temperature rise caused by thermal interference.
According to the teaching of Japanese Laid-Open Patent Application No. 2000-114656, in order to keep the etching bottom in the flat part in the circumference of the laser array from reaching the lower semiconductor Bragg reflector, the dummy elements may be arranged in the whole wafer to make small the difference Δd between the etching depth in the central array part and the etching depth in the flat part in the circumference of the array part.
If the flat part is not eliminated, the etching bottom reaches the lower semiconductor Bragg reflector so that AlAs is oxidized. Thus, it is necessary to arrange the dummy elements in the whole wafer.
However, if the dummy elements are arranged in the whole wafer, the area which is to be etched is decreased. This causes monitoring of oxidization (plasma emission spectrometry, optical reflective index analysis, etc.) to be difficult to perform. Moreover, if the dummy elements are arranged in the whole wafer, unevenness appears on the surface of the laser array and the probability of wire cut-off is raised. Furthermore, it is needed to form the wire bonding pads for implementation. However, if unevenness exists under the bottom of the bonding pads, the mesa structure may be damaged at the time of wire bonding, which causes a faulty surface-emitting laser array to be produced.